Pressure control unit for hydraulic systems



1966 J. MERCIER PRESSURE CONTROL UNIT FOR HYDRAULIC SYSTEMS 5Sheets-Sheet 1 Filed Jan. 51, 1965 6 1 M 7 R v k 9 LT L W C mm N, W z 2mm W 00 c mm a J Q lo Z q w. m: 3 Q R 4 k a 5 8 61 z- On h .NN NN p d .3mm m. mm 3 v N n mm mm Fm 2 v S mm 3 C 1 m c D wwm w I. p L NIW- mm IQ.Q 2 S 9 5 Sheets-Sheet 2 J- MERCIER Dec. 27, 1966 PRESSURE CONTROL UNITFOR HYDRAULIC SYSTEMS Filed Jan. 31, 1963 R R a M c N R 9 IN! I I lll 5E I111 .1 M om /Y O a w z 3 m E :S T W N 6 8 9 2 Q 2 PM m mm F N 0E E 8mm 5 H k 3 m 6 a? on 3 mm 5 L v 4% mm JWV 2 8 \I I {I W B 8 Q Q i on 2Dec. 27, 1966 J MERGER 3,294,104 I PRESSURE CONTROL UNIT FOR HYDRAULICSYSTEMS Filed Jan. 31, 1965 5 Sheets-Sheet 5 Dec. 27, 3966 J. MERCIER3,294,104

PRESSURE CONTROL UNIT FOR HYDRAULIC SYSTEMS Filed Jan. 51, 1963 5Sheets-Sheet 4 MVENTJR 71-79 Wife/Er? QQIIJWVM Dec. 27, 1966 J. MERCIER3,294,104

PRESSURE CONTROL UNIT FOR HYDRAULIC SYSTEMS Filed Jan. 31, 1963 5Sheets-Sheet 5 United States Patent l 3,294,104 PRESSURE CONTROL UNITFOR HYDRAULIC SYSTEMS Jean Mercier, 1185 Park Ave., New York, N.Y.143923 Filed Jan. 31, 1963, Ser. No. 255,401 Claims priority,appiication France, Feb. 6, 1962,

,088 10 Claims. (Cl. 1371tl8) This invention relates to the art ofcontrol units, more particularly of the type to retain the pressure in ahydraulic system within relatively narrow predetermined limits.

As conducive to an understanding of the invention, it is noted thatwhere discharge valves for hydraulic systems operate to relieve thepressure when it reaches a predetermined amount and then close when thepressure has fallen to a lower predetermined amount and the rangebetween such two pressures is great, the utilization of such a dischargevalve is extremely limited, for it may not be capable of use in systemsin which the operating pressure must be maintained within closetolerances.

It is accordingly among the objects of the invention to provide acontrol unit which will relieve the pressure in a hydraulic system whenit reaches a predetermined amount and which will permit recharge of thesystem to increase the pressure at an amount but slightly less than thepressure at which the system discharges, which control unit isrelatively simple in construction and automatic in operation and is notlikely to become deranged even with long use.

According to the invention from its broader aspect, the control unitincludes a discharge valve which is controlled by a pilot valve, andresilient means are provided which react against the pilot valvenormally to retain the latter in closed position at which time thedischarge valve will also be closed. As the pressure in the system risestoward a first predetermined value, the force exerted by the resilientmeans against the pilot valve will be reduced and when the systempressure reaches such first predetermined value the pilot valve will beactuated to effect opening of the discharge valve. When the dischargevalve has opened to relieve the pressure in the system, the forceexerted by said resilient means against the pilot valve will beincreased whereby when the system pressure has fallen to a secondpredetermined value, the pilot valve will again be actuated to eifectclosing or" the discharge valve.

In the accompanying drawings in which is shown one or more of variouspossible embodiments of the several features of the invention,

FIG. 1 is a view showing one embodiment of the invention partly inlongitudinal cross section,

FIGS. 2 and 3 are views similar to FIG. 1 showing the embodiment of FIG.1 in various stages of operation, and

FIGS. 4 and 5 are views similar to FIG. 1 of other embodiments of theinvention.

Referring now to the drawings, in the embodiment shown in FIGS. 1 to 3the control unit which includes a main discharge valve V and a controlvalve CV is incorporated into a hydraulic system including a pump N, areservoir 10, a pressure accumulator U and a use circuit L.

The pump N has its inlet end connected to the reservoir and its outletend connected through line 11 to junction 12 which in turn is connectedto lines 13 and 14. The line 13 is connected to the pressure inlet port13 of the main discharge valve V. The line 14 is connected throughone-way valve 15 to junction 14, the valve 15 permitting flow only fromline 14 to junction 14. The junction 14 is connected by line 17 tocontrol port 17' 3,294,104 Patented Dec. 27, 1966 of control valve CVand by lines 18 and 19 to the fluid port 18 of pressure accumulator Uwhich may be of any conventional type such as that shown in US. PatentNo. 2,931,392, and to the use circuit L to be operated by the hydraulicsystem.

The main discharge valve V has a closed hollow cylindrical body 22. inwhich a piston 23 is slidably mounted. The piston 23 has a cylindricalportion 24 and a conical portion 25. The cylindrical portion 24 iscentered in the bore of the cylindrical body 22 by means of spacedparallel ribs 24' on the outer surface of the cylindrical portion andextending parallel to the longitudinal axis thereof, the spaces betweenthe ribs 24' defining passages 26. The conical portion 25 of the piston23 is designed to seat on the mouth 28 of a line 29 which extends intothe reservoir Ill, said mouth 28 defining a discharge port.

The piston is normally retained in closed position with respect to theseat 28 'by means of a coil spring 3% positioned in the bore of thecylindrical body 22 and reacting against the piston 23, as is clearlyshown in FIG. 1. The portion of the bore of the cylindrical body 22between the piston and the end of the discharge valve defines a controlchamber M and the portion of the bore between the cylindrical portion 24of the piston and the seat 28 when the piston is in closed positiondefines a chamber 33 in communication with the port 13'.

The control valve CV comprises an elongated body portion 35 having twocylindrical bore portions 36, 37 therein in communication with eachother through a passageway 33 of relatively small cross sectionextending through a wall 38' between the bores 36, 37.

Slidably mounted in the bore 36 is a piston 39 encompassed by a sealring 39' and which defines a control chamber C on the outer side of thepiston and a discharge chamber D on the inner side of the piston. Theouter end of bore 36 is closed by means of a threaded plug 43 which hasan axial 'bore 43' therethrough, the outer end of which defines thecontrol port 17'. The discharge chamber D is connected throughtransverse passageway 40, port 49 and line 42 to the reservoir 16.

The end of the body portion 35 associated with the bore 37 is of reducedouter diameter as at 52 and is externally threaded to receive aninternally threaded control cap 51, the latter having an axial bore 50through the end wall thereof.

Slidably mounted in the bore 37 is an adjustment piston J encompassed bya sealing ring I. The piston J has a stem 46 of reduced diameter as at4-8 at its outer end defining an annular shoulder 47, the reduceddiameter portion 48 being externally threaded as at 49 at its outer endand extending through the bore 50 of the cap 51, a nut 53 and a lock nut54 being screwed on such threaded end 49. The nut 53 and the shoulder 47are arranged on opposite sides of the end wall of the cap 51 and definetwo stops which cooperate with said end wall to determine two extremepositions of the stroke of the piston I in the bore 37.

The piston I is normally retained in its innermost position in the bore37 by means of a coil spring 55, compressed between the piston and theend wall of the cap 51. An additional spring R, which is weaker than thespring 55, is also positioned in bore 37 on the other side of the pistonI, the spring reacting against the piston J and a ball 57 located inbore 37 and of diameter less than the diameter of said bore 37 so thatit is freely rnovable therein, said ball 57 reacting against a pilotvalve P. It is to be noted that the ball 57 does not constitute a tightbarrier in the bore 37, but permits leakage of fluid past the sameduring operation of the unit.

The pilot valve P is a substantially frusto-conical member having itstapering wall adapted to be moved against the end 59 of the passage 38which defines a seat for the pilot valve. The portion of the bore 37between ball 57 and the Wall 38' defines a chamber A which is connectedby transverse passageway 34 and line 34 to a port 22' leading into thechamber M of the discharge valve V.

The pilot valve P has a stem 60 secured thereto which extends axiallythrough the passage 38 with play since the diameter of the stem 60 isless than that of the passage 38 and the end 61 of the stem 60 restsagainst the bottom of a conical cavity 62 provided in the inner end ofthe piston 39.

It is to be noted that when the piston I is in its extreme position tothe left, referring to FIG. 1, so that the shoulder 47 is restingagainst the end wall of cap 51, the

coil spring R will have a predetermined tension which may be adjusted byrotation of the cap 51 for the purposes hereinafter set forth.

When the piston I is in its innermost position in bore 37 or its extremeposition to the right, determined by the abutment of nut 53 against theouter surface of the end wall of cap 51, the coil spring R will have adifferent and greater predetermined tension and this tension isadjustable by rotation of the nuts 53, 54.

In the operation of the equipment shown in FIG. 1, when the pump N isenergized, fluid under pressure will be forced through line 13, port 13into chamber 33 of discharge valve V and then through the passageways 26into chamber M and through the line 34' into the chamber A of thecontrol valve CV to increase the pressure in such chamber A. Due to thefact that the cross sectional area of the upper end of the piston 24defined by the wall 31 is greater than the useful cross sectional areaof the conical portion 25 determined by the diameter of seat 28 and inaddition, due to the force exerted by coil spring 30, the dischargevalve V will be in its closed position as shown.

Fluid from the pump N will also flow through line 14, one-way valve 15to junction 14' and thence through line 17 into control chamber C of thecontrol valve and through line 18 to charge the accumulator U and alsothrough line 19 to the hydraulic circuit to be operated.

As long as the pressure in the system remains below a predeterminedamount the control unit will be in the position shown in FIG. 1, i.e.,the discharge valve V will be in closed position and the pilot valve Pwill be seated against its seat 59.

As the pressure in chamber A increases, the fluid will flow past theball 57 and react against the right hand face of piston J and as thepressure in chamber A rises above a predetermined amount based on thetension of coil spring 55, the piston I will be moved to the left untilthe shoulder 47 abuts against the end wall of the cap 51 as shown inFIG. 2. At this time the coil spring R will have extended, as shown inFIG. 2, with the pilot valve P still in closed position.

With further build up of pressure in the system, the pressure in chamberC will rise and exert a force against the piston 39 to tend to move thelatter to the left. When such pressure has reached a predetermined valuesufficient to overcome the force exerted by spring R and the pressure inchamber A reacting against pilot valve P, the piston 39 will move to theleft and through the valve stem 60 to move the pilot valve P off itsseat 59. It is to be noted that the effective area of piston 39 againstwhich the fluid pressure will react is much greater than the effectivearea of pilot valve P.

It is apparent that by adjusting the cap 51, the tension on spring R maybe varied and hence the pressure at which the valve P. will move off itsseat may be varied.

. When valve P moves off its seat 59, the chamber A will be placed incommunication with the reservoir 10 through the passageway 38 and line42.

As a result of the opening of chamber A, the fluid pressure in bore 37will be reduced and hence the piston J will move to the right under theurging of coil spring 55 until the nut 53 abuts against the outersurface of the end wall of the cover 51. Due to the relatively smallcross section of the passageway 38, a dashpot effect will be createdwhich will avoid any sudden movement. At this time the coil spring Rwill have again been compressed as shown in FIG. 3.

The opening of chamber A as above described, will also cause thepressure in chamber M of discharge valve V to drop and since the pump Nis still operating, forcing fluid under pressure into chamber 33, thepiston 23 will be moved away from its seat 28 providing a path for thefluid from the pump N through line 29 back into the reservoir 10.

At this time, since the pressure in the accumulator U will be reactingagainst the one-way valve 15, the latter will be in closed position.

At this time, the pressure in the chamber C, due to the charge inaccumulator U, will still be suflicient to retain the pilot valve P inopen position even though the spring R has again been compressed. Thisis clearly shown in FIG. 3. As the accumulator U feeds fluid underpressure to the system, the pressure will drop in line '17 and hence inchamber C of control valve CV. When the pressure has dropped below apredetermined value so that the force exerted against the piston 39 isnot able to overcome the force of the compressed spring R, the pilotvalve P will again be moved to the right against its seat 59 to closethe passageway 38, as shown in FIG. 1. The discharge valve V will againclose due to the build up of pressure in chamber M and permit the pump Nto feed fluid under pressure through the line 15 into the accumulator,the use system L and the control chamber C in the manner previouslydescribed.

It is to be noted that the opening of the pilot valve P which in turncauses the discharge valve V to open to prevent rise of pressure in thesystem above a predetermined value, takes place after the spring R hashad its tension reduced as shown in FIG. 2. It is also to be noted thatwhen the pilot valve P moves from the open position shown in FIG. 3 tothe closed position shown in FIG. 1, the spring R is exerting greatertension.

As a result of the arrangement shown, the difference between thepressure at which the discharge valve V will open and the pressure atwhich it will close may be relatively small, in some cases up to threeor four percent of the desired pressure instead of the fifteen or twentypercent which is customarily necessary in order for the conventionaldischarge valves to function.

As a result of the reduction of the pressure drop required betweendischarge and recharge of the accumulator, the system herein describedproduces much greater sensitivity in operation than is presentlyavailable which is desirable and sometimes necessary in numerousapplications.

The embodiment shown in FIG. 4 is similar in many respects to theembodiments shown in FIGS. 1 to 3 and corresponding parts have the samereference numerals.

In the embodiment shown in FIG. 4, the control valve CV has a bodyportion 65 with two longitudinally aligned cylindrical bores 66 and 67,the bore 66 being of smaller diameter than the bore 67. Slidably mountedin the bores 66, 67 is the adjustment piston I which has portions 69 and70 slidable in the bores 66 and 67 respectively, said portion 69 havinga reduced diameter extension 69' which is screwed into an axial recessin the cylindrical portion 70 of the piston J, the larger diameterportion 70 defining an annular shoulder 68.

Positioned in the outer end of bore 66 is a cylindrical plug 72encompassed by an annular gasket 73, the end of the plug reactingagainst the end wall of a cup-shaped cap 74 screwed on the externallythreaded end 75 of the body portion 65. The outer end of bore 67 isclosed by means of a cup-shaped plug which has an externally threadedportion 86 screwed in the correspondingly internally threaded endportion of the bore 67 as is shown in FIG. 4.

Positioned in the bore 66 is a spring R compressed between the inner endof plug 72 and a ball 77 of diameter less than the diameter of the bore66. The ball 77 is urged by the spring R against a pilot valve P toretain the latter against its seat 79, the bore 66 being connected byline 42 to the reservoir 10. The plug 72 and the end of piston J definea chamber D. A chamber C is located at the end of the axial recess 80 inpiston portion 70 and chamber C is connected through lateral passageway70 in portion 70 to an elongated annular groove 76 in the periphery ofthe piston portion '70, appropriate seal rings being provided at eachend of the groove 76 to prevent leakage. The annular groove 76 is incommunication through a transverse passageway 76 with line 17 whichleads to junction 14. The inner end of the bore 67 of the body portion65 is of reduced diameter defining an annular chamber A and chamber A isconnected through transverse passageway 34 and line 34' to the port 22of discharge valve V which leads into the chamber M. The reduceddiameter portion of bore 67 defines a shoulder 67' against which theshoulder 68 defined by the larger portion 70 of the piston I may abut tolimit the inward movement of such piston I under the urging of coilspring 55 which is compressed between the end wall of plug 85 and theadjacent end of the piston I. It is to be noted that the plug 85 has arecess in which the spring 55 is contained and the inner end 85 of theplug 85 defines a stop for the piston J, as is shown in FIG. 4 to limitthe outward movement of the piston to the right, such movement beingcontrolled by the setting of the plug 85.

The portion 69 of the piston I has an axial bore 80' therethroughleading into the chamber C in portion 70, the axial bore 80 having atransverse passageway 84 which is in communication with the chamber A.The pilot valve P has a stem 83 of smaller diameter than the bore 80'and which extends thereinto; a second stem 83a is also positioned in thebore 80 and is also of smaller diameter than the latter and mounts atits outer end located in the chamber C, a valve head 82 designed to seaton the enlarged diameter end 81 of bore 80. The lengths of the stems 83and 83a are such that when one of the valve heads P or 82 is seated, theother will be spaced from its seat.

It is to be noted that the coil spring 55 is stronger than the spring Rand normally the piston I will be moved to the left so that its shoulder68 will be against the shoulder 67.

The operation of the embodiment shown in FIG. 4 is similar in manyrespects to the operation of the embodiment shown in FIGS. 1 to 3. Itwill be noted that the passages 26 provided in the discharge valve V ofFIGS. 1 to 3 are not required in the embodiment of FIG. 4.

In the operation of the unit shown in FIG. 4, as the pump N forces fluidunder pressure into the system, such fluid under pressure will enter thechamber 33 through line 13 and also flow through line 14 to charge theaccumulator U and also enter the chamber C and pass through suchchamber, through bore 80 and passageway 84 into chamber A. The fluidunder pressure in chamber A will pass through passageway 34 and line 34into the cham ber M of the discharge valve V reacting against thesurface 31 of the piston 23 therein and this force together with theforce of the spring will retain the piston 23 in closed position againstits seat 28, even against the pressure in chamber 33. At this time thespring R will be in its compressed position and the piston I will be inits position to the left against shoulder 67.

As the pressure in the system builds up, there will be an increase inthe pressure in chambers C and A. Such increase in pressure in chamber AWill force the piston J to the right compressing the spring 55, movementof the piston I being facilitated 'by the port 86' in plug which servesas a vent. The piston I will move to the right until it abuts againstthe end 85 of plug 85 and the tension on the spring R will be reduced.With further increase in pressure to a predetermined value which alsowill be applied through bore 80 against the seated pilot valve P, sincethe force retaining the latter seated has been reduced by reason of theextension of the spring R when piston J moves to the right, such pilotvalve P will move oif its seat 79. As a result, the chambers C and Awill discharge through bore 80, bore 66 and line 42 into the reservoirthereby permitting the piston to be moved to the left by reason of thetension of coil spring 55, and the spring R will again be compressed.However, as the pressure in chamber C reacting against the effectivearea of valve head 82 will be greater than that in .bore 66 reactingagainst the effective area of valve P, the valve head 82 will now beseated and through the stems 83a and 83, the valve P will remain spacedfrom its seat. Reduction in fluid pressure in chamber A will causecorresponding reduction of pressure in chamber M and hence the pre surein chamber 33 will overcome the force exerted by the coil spring 30 sothat the piston 23- of discharge valve V will move away from its seat 28thereby permitting the pump N to discharge into the reservoir 10.

In the manner previously described with respect to the embodiments inFIGS. 1 to 3, during use of the accumulator U when the pressure inchamber C drops below a predetermined amount, the force exerted by thenow comressed spring R will be suflicient to move the pilot valve P backon to its seat 79 to close the bore 80' and the operation will again berepeated as the pressure varies.

In the embodiment shown in FIG. 5 the control unit comprises a maindischarge valve V which has a body portion with a bore therein in whicha piston is slidably mounted. The piston 90 has an elongated anulargroove 93 between the ends thereof defining two heads 91 and 92. Theannular chamber defined by groove 93 is connected by line 13 to theoutlet end of a pump N, the inlet end of which is connected to reservoir10-.

Between the piston head 92 and the adjacent end of the body 90, acontr-olchamber M is defined which is connected by line 95 to port 95 ofcontrol valve CV. The chamber M is formed by a reduced diameter portionof the bore of the body 90 which forms a shoulder M and the piston 90 isnormally retained against shoulder M by means of a coil spring 96positioned in the bore of the discharge valve and reacting against thehead 91 of the piston. With the piston seated against the shoulder M, adischarge port 98 in valve body 90 is closed by'head 91, said port 98being connected by line 99 to the reservoir 10. The port 98 is closedwhen the pressure in the chamber M is below a predetermined value. Whenthe pressure in chamber M rises to a predetermined amount, the piston 90will be moved to the left from the piston shown in FIG. 5 against thetension of the coil spring 96 and at such time the pressure inlet port13' and discharge port 98 will be connected through chamber 93, causingthe pump N to force fluid back into the reservoir 10.

The control valve CV has a body portion 100 with an axial bore 101. Anadjustment piston J is slidably mounted in the bore 101 and has areduced diameter portion which extends through an opening in the endwall of a cap screwed on the end of the body 100-. The reduced diameterend of piston J is threaded to receive nuts 104 and such reduceddiameter portion defines an annular shoulder 103 at its root end, saidshoulder 103 and nuts 104 forming two stops which determine the inwardand outward position of the piston I with respect to the end wall of cap105.

The cap 105 and the adjacent portion of the body portion 100 on which itis screwed, have complementary shoulder portions 100 which serve tolimit the inward 7 position ofthe cap. A chamber 111 is defined betweenthe end of the body portion 100 and the end wall of the cap 105.

The bore 101 of the body portion 100 is in communication throughtransverse passageway 42' with line 42, leading to the reservoir 10. Thepiston J and a plug 106 positioned in the outer end of the bore of themember 100 define a chamber D therebetween.

As is clearly shown in FIG. 5, the plug 106 has three parts, 120, 121and 122. The 'part 120, which is adjacent the chamber D, is seatedagainst an annular shoulder 123 and is encompassed by a suitable sealingring. The part 121 is interposed between the part 120 and the part 122,the part 122 has an externally threaded portion which coacts with acorrespondingly internally threaded portion 107 in the bore of the bodyportion 100' and hence when the part 106 is screwed into place it willproperly position the parts 120 and 121. The part 120 has an axial bore113 which leads into an axial bore in part 121 defining a chamber A, thediameter of chamber A being greater than that of bore 113 and forming anannular shoulder 112. The chamber A is connected through transversepassageways 124 and 125 to port 95' which in turn is connected by line95 to the chamber M of the discharge valve. In addition, the chamber Ais connected through passage 126, and an annular groove 127 topassageway 108 which is connected to the chamber 111.

A pilot valve P in the form of a ball of smaller diameter than thechamber A, is positioned in the latter and can be moved to the rightagainst the seat 110 defined at the inner end of passageway C in part122, which passageway is connected by line 17 to junction 18' or againstthe seat 112 of a larger cross section than seat 110 and formed at theouter end of bore 113 which leads to chamber D. An adjustment spring Ris positioned in the bore 101 between the piston J and a ball 114 ofsmaller diameter than the diameter of bore 101 and the ball 114 reactsagainst a head 115' to which valve stem 115 is connected, the latterextending with play through the bore 113 and reacting against the ballvalve P.

In the embodiment of FIG. 5, the adjustment of the calibration of thespring R is elfected by movement of the piston J without the resilientmeans such as spring 55 of FIGS. 1 to 4, for example.

When the system of FIG. 5 is initially started the spring R will beextended so that the valve P will be against opening 110 to close thelatter and the annular shoulder 103 of piston J will be seated againstthe end wall of cap 105. At this time the chamber M of the dischargevalve V is connected to the reservoir through line 95, port 95',passageways 125, 124, chamber A, bore 113, chamber D and line 42.Consequently, no pressure will be applied to chamber M and the piston90' will be in the position shown, closing the discharge port 98. Atthis time also, the chamber 111 of the control valve CV will beconnected to the reservoir 10 through passageway 108, passageways 127and 126, chamber A, bore 113, chamber D and line 42. As a result, therewill be no fluid pressure in the chamber 111 and hence the piston I willbe in its left hand position with the spring R extended.

As the pressure in the system increases it will rise in line 17 andhence the valve P will be moved to the left away from its seat 110against the relatively small force exerted -by the extended spring R andsuch valve P will seat against the valve seat 112, closing the bore 113.This will occur when the pressure in the system has reached apredetermined value. At such time fluid under pressure will flow throughline 17 into chamber A and thence through port 95' and line 95 intochamber M of the discharge valve V to move the piston 90 thereof to theleft to connect ports 13' and 98 so that the pump N will discharge intothe reservoir 10. In addition, fluid under pressure will flow fromchamber A through passageway 108 into chamber 111 and react against theshoulder 103 of the piston J, moving the piston to the right until thenut 104 strikes the outer surface of the end wall of the cap 105. As aresult, the spring R will be compressed ready for the recharging cycle.

As the pressure in line 17 decreases due to the utilization of theaccumulator, the pressure in chamber A will drop tnd hence the pilotvalve P will be moved oif the seat 112 by reason of the force exerted bythe compressed ooil spring R and again close the seat 110. The operationwill then repeat in the manner above set forth.

With the equipments above described, it is apparent that once thedischarge valve has opened when the system pressure has reached apredetermined amount, to prevent further increase in pressure, since theresilient means R which controls the pilot valve P will have had itsforce materially increased, a slight decrease in the pressure in thesystem, will cause a reversal of operation so that the discharge valvecan then close to permit the pressure again to build up. It is thereforeapparent that the range between discharge and recharge can be maintainedwithin small limits which may be extremely important if not necessary inmany applications of hydraulic controldevices.

As many changes could be made in the above constructions and manyapparently widely different embodiments of this invention could be madewithout departing from the scope of the claims, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings shall be interpreted as illustrative and not in a limitingsense.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

1. A pressure control unit for maintaining the pressure in a hydraulicsystem within predetermined limits, comprising a normally closeddischarge valve having a discharge port, a valve member adapted to closesaid discharge port, said discharge valve having a pressure inlet portassociated with one end of said valve member and a control portassociated with the other end of said valve member, means in saiddischarge valve providing communication between said inlet port and saidcontrol port, a control valve comprising a body portion having apressure chamber, a discharge chamber and a control chamber, saidpressure chamber having a port connected to the control port of saiddischarge valve, said discharge chamber having a discharge port and saidcontrol chamber having a pressure inlet port, a passageway in saidcontrol valve providing communication between said pressure chamber andsaid discharge chamber, said passageway having a port in said pressurechamber defining a valve seat, a pilot valve controlling said seat, apiston in said pressure chamber, a first coil spring operativelyinterposed between said piston and said pilot valve normally to retainthe latter on said seat to close said passageway,

a second coil spring of greater force than said first coil springreacting against said piston in the opposed direction to said first coilspring, a piston in said control chamber, rigid means extending throughsaid passageway and coacting with said second piston and said pilotvalve to urge the pilot valve away from said seat, whereby when thepressure in said pressure chamber and said control chamber rises to apredetermined value, the piston in said pressure chamber will be movedin direction to reduce the force of said first coil spring against saidpilot valve and the piston in said control chamber will move indirection to cause said rigid means to move the pilot valve off itsseat, thereby permitting flow of fluid from said pressure chamberthrough said passageway into the discharge chamber, to reduce thepressure in said control port of the discharge valve to permit openingof the latter by the pressure applied to the inlet port thereof, saidfirst coil spring being compressed when the pressure 9 in said pressurechamber is reduced by the movement of the first piston by the secondcoil spring to seat said pilot valve when the pressure in said controlchamber has fallen below a predetermined amount.

2. The combination set forth in claim 1 in which a pressure accumulatoris provided having a fluid port connected to said pressure inlet port ofsaid control chamber and to said pressure inlet port of said dischargevalve, a one-way valve prevents flow from said accumulator to said inletport of said discharge valve and a source of fluid under pressure isconnected to said inlet port of said discharge valve.

3. The combination set forth in claim 1 in which a cap is mounted onsaid body portion, said pressure chamber piston has .a stem extendingthrough said cap, and stop members are secured respectively to the outerend of the stem and to a portion of the stem in said bore, said stopmembers abutting against said cap to define the two extreme positions ofsaid piston in said bore.

4. The combination set forth in claim 3 in which the cap is rotatablymounted on said body portion and the stop member on the outer end of thestem is adjustably mounted thereon whereby the stroke of said piston maybe adjusted.

5. A pressure control unit for maintaining the pres sure in a hydraulicsystem within predetermined limits comprising a normally closeddischarge valve having a normally closed discharge port, a valve memberadapted to close said discharge port said discharge valve having apressure inlet port associated with one end of said valve member and acontrol port associated with the other end of said valve member, acontrol valve having a pressure inlet port, connected to said controlport a pilot valve in said control Valve operatively connected to saidpressure inlet port and operatively controlling the movement of thevalve member of said discharge valve, means controlled by the pressurein said hydraulic system reacting against said pilot value to urge thelatter to open position, first resilient means initially under maximumcompression, reacting against said pilot valve and retaining the latterin closed position, pressure responsive resiliently controlled means toeffect reduction in the force exerted by said first resilient meansagainst said pilot valve as the hydraulic system pressure rises toward apredetermined value, whereby when said predetermined pressure isobtained so that the force exerted by the means controlled by thepressure in said hydraulic system exceeds the force exerted by thepressure in said pressure chamber together with said first resilientmeans, said pilot valve will open thereby to increase the force exertedby said first resilient means against said pilot valve, whereby whensaid system pressure has fallen to a second predetermined value, thepilot valve will be closed by said first resilient means to effectclosing of said discharge valve.

6. The combination set forth in claim 5 in which said resilientlycontrolled means comprises a pressure responsive piston which reactsagainst said first resilient means, said piston being movable betweentwo predetermined positions.

7. A control unit for maintaining the pressure in a hydraulic systemwithin predetermined limits, comprising a control valve having a bodyportion with a bore therein, a piston slidably mounted in said bore andmovable between two extreme positions, a chamber in said bore having twoopposed walls a pressure inlet port leading into said chamber one endof, said piston forming one wall of said chamber a first coil springreacting against the other end of said piston, a passageway extendingthrough the other wall of said chamber, a pilot valve associated withthe end of said passageway in said chamber, a second coil spring in saidchamber reacting against the first end of said piston and said valve andnormally urging said valve to close said passageway, said first coilspring being stronger than said second coil spring to urge said pistonto one of its extreme positions to compress said second coil spring, adischarge port in said body portion in communication with the other endof said passageway, fluid pressure operated means in said boreoperatively connected to said pilot valve, a discharge valve having apressure inlet port, a discharge port and a control chamber, a piston insaid discharge valve in communication with said control chamber andnormally retaining the discharge port closed, a line connecting saidcontrol valve chamber and said discharge valve inlet port through saiddischarge valve and means to apply fluid under pressure to said controlvalve chamber and to said fluid pressure operated means.

8. The combination set forth in claim 7 in which said fluid pressureoperated means comprises a pressure responsive piston in said bore onthe other side of said other wall, a pressure chamber in said bore, saidpiston forming a wall of said pressure chamber, a pressure inlet portleading into said pressure chamber to which the fluid under pressure insaid hydraulic system is connected, and an actuating rod extendingthrough said passageway and abutting at its ends against said pilotvalve and said pressure chamber piston.

9. The combination set forth in claim 8 in which said discharge port insaid body portion is in communication with the portion of the pistonbore between the wall through which the passageway extends and saidpressure chamber piston.

10. The combination set forth in claim 7 in which a restricted fluidpassageway is provided between said discharge valve pressure inlet portand said discharge valve control chamber, resilient means in saiddischarge valve control chamber normally urges said piston to closedposition, opposed portions of the piston are exposed to the pressure inthe control chamber of the discharge valve and to the pressure throughthe pressure inlet port of the discharge valve and the area of thepiston exposed to the pressure in the control chamber of the dischargevalve is greater than that exposed to the pressure through the pressureinlet port of the discharge valve.

References Cited by the Examiner UNITED STATES PATENTS 1,998,553 4/1935Maxson 137-108 2,274,663 3/1942 Brisbane 137488 2,609,832 9/1952 Smith137489 WILLIAM F. ODEA, Primary Examiner.

ISADOR WEIL, Examiner.

D. ZOBKIW, Assistant Examiner.

1. A PRESSURE CONTROL UNIT FOR MAINTAINING THE PRESSURE IN A HYDRAULICSYSTEM WITHIN PREDETERMINED LIMITS, COMPRISING A NORMALLY CLOSEDDISCHARGE VALVE HAVING A DISCHARGE PORT, A VALVE MEMBER ADAPTED TO CLOSESAID DISCHARGE PORT, SAID DISCHARGE VALVE HAVING A PRESSURE INLET PORTASSOCIATED WITH ONE END OF SAID VALVE MEMBER AND A CONTROL PORTASSOCIATED WITH THE OTHER END OF SAID VALVE MEMBER, MEANS IN SAIDDISCHARGE VALVE PROVIDING COMMUNICATION BETWEEN SAID INLET PORT AND SAIDCONTROL PORT, A CONTROL VALVE COMPRISING A BODY PORTION HAVING APRESSURE CHAMBER, A DISCHARGE CHAMBER AND A CONTROL CHAMBER, SAIDPRESSURE CHAMBER HAVING A PORT CONNECTED TO THE CONTROL PORT OF SAIDDISCHARGE VALVE, SAID DISCHARGE CHAMBER HAVING A DISCHARGE PORT AND SAIDCONTROL CHAMBER HAVING A PRESSURE INLET PORT, A PASSAGEWAY IN SAIDCONTROL VALVE PROVIDING COMMUNICATION BETWEEN SAID PRESSURE CHAMBER ANDSAID DISCHARGE CHAMBER, SAID PASSAGEWAY HAVING A PORT IN SAID PRESSURECHAMBER DEFINING A VALVE SEAT, A PILOT VALVE CONTROLLING SAID SEAT, APISTON IN SAID PRESSURE CHAMBER, A FIRST COIL SPRING OPERATIVELYINTERPOSED BETWEEN SAID PISTON AND SAID PILOT VALVE NORMALLY TO RETAINTHE LATTER ON SAID SEAT TO CLOSE SAID PASSAGEWAY, A SECOND COIL SPRINGOF GREATER FORCE THAN SAID FIRST COIL SPRING REACTING AGAINST SAIDPISTON IN THE OPPOSED DIRECTION TO SAID FIRST COIL SPRING, A PISTON INSAID CONTROL CHAMBER, RIGID MEANS EXTENDING THROUGH SAID PASSAGEWAY ANDCOACTING WITH SAID SECOND PISTON AND SAID PILOT VALVE TO URGE THE PILOTVALVE AWAY FROM SAID SEAT, WHEREBY WHEN THE PRESSURE IN SAID PRESSURECHAMBER AND SAID CONTROL CHAMBER RISES TO A PREDETERMINED VALUE, THEPISTON IN SAID PRESSURE CHAMBER WILL BE MOVED IN DIRECTION TO REDUCE THEFORCE OF SAID FIRST COIL SPRING AGAINST SAID PILOT VALVE AND THE PISTONIN SAID CONTROL CHAMBER WILL MOVE IN DIRECTION TO CAUSE SAID RIGID MEANSTO MOVE THE PILOT VALVE OFF ITS SEAT, THEREBY PERMITTING FLOW OF FLUIDFROM SAID PRESSURE CHAMBE THROUGH SAID PASSAGEWAY INTO THE DISCHARGECHAMBER, TO REDUCE THE PRESSURE IN SAID CONTROL PORT OF THE DISCHARGEVALVE TO PERMIT OPENING OF THE LATTER BY THE PRESSURE APPLIED TO THEINLET PORT THEREOF, SAID FIRST COIL SPRING BEING COMPRESSED WHEN THEPRESSURE IN SAID PRESSURE CHAMBER IS REDUCED BY THE MOVEMENT OF THEFIRST PISTON BY THE SECOND COIL SPRING TO SEAT SAID PILOT VALVE WHEN THEPRESSURE IN SAID CONTROL CHAMBER HAS FALLEN BELOW A PREDETERMINEDAMOUNT.